I have to monitor some crontabs and they support email notification on errors. I don't want to manage and maintain another mailserver, therefore I installed postfix and configured it as relay. So the system is able to send mails, but another mailserver is used. The new postfix setup accepts only local SMTP connections and port 25 is not accessible from the outside.

Modoboa is a nice all-in-one email package. Sadly it lacks configuration possibilities for certificates and the default config uses just self signed certificates. This tutorial explains how to create and configure lets encrypt for your mail server.